New high resolution geochemistry of Lower Jurassic marine sections in western North America: A global positive carbon isotope excursion in the Sinemurian?

Abstract

Deposition of black shale facies, one of the main contributors for hydrocarbon production, is commonly assumed to be linked to Oceanic Anoxic Events (OAEs), which are supposed to be generated during warm palaeoclimates. This assumption could bias the exploration for hydrocarbon source rocks preferentially towards sediments deposited under warm palaeonvironments, as a preferential guide for hydrocarbon exploration. As a consequence, the establishment of the links between palaeotemperature and the formation of organic-rich deposits is of primary importance. For this purpose, the Upper Sinemurian, Pliensbachian and Lower Toarcian (Lower Jurassic) deposits of the Asturian Basin in northern Spain, including more than 100 m thick succession containing organic-rich and black shale deposits, have been studied. Correlation between palaeoclimatic data, previously obtained from oxygen isotopes, and the TOC content reveals that black shales and organic-rich sediments were deposited not only during warming intervals but also during a prominent cooling event that occurred at the Late Pliensbachian. This cooling period has been pointed out as one of the main intervals to have developed ice caps in the poles during the Jurassic. On the contrary, no black shales were generated during the postulated Early Toarcian OAE, which coincides with a superwarming interval. Additionally, the study of the facies cycles and the measurement of gamma-ray in outcrops and its correlation with TOC content provides data on the use of natural radioactivity and sea level changes as a proxy for preliminary organic matter richness evaluation.Keywords: Organic-rich deposits, Climatic change, Oxygen isotopes, Carbon isotopes, Gamma ray, Sea level changes ResumenNormalmente se asume que la sedimentación de las facies de black shales, uno de los principales contribuyentes a la generación de hidrocarburos, está ligada a los Eventos Oceánicos Anóxicos (OAEs) y que éstas facies se formaron durante paleoclimas cálidos. Esta suposición puede orientar a la exploración de rocas madre de hidrocarburos preferentemente hacia los sedimentos depositados bajo paleoambientes cálidos. En consecuencia, el establecimiento de la relación entre paleotemperaturas y la formación de depósitos ricos en materia orgánica resulta de gran importancia para encontrar argumentos sobre este tema, aplicables a la exploración de hidrocarburos. Con este propósito se han estudiado los sedimentos del Sinemuriense Superior, Pliensbachiense y Toarciense Inferior (Jurásico Inferior) de la Cuenca de Asturiana, que incluyen más de 100 m de espesor de depósitos conteniendo facies con materia orgánica y black shales. La correlación entre los datos paleoclimáticos, obtenidos previamente a partir de isótopos de oxígeno, y el contenido de Carbono Orgánico Total (TOC) revela que los black shales y los sedimentos ricos en materia orgánica se depositaron no solo durante los intervalos cálidos sino también durante un destacado evento frío que se desarrolló durante el Pliensbachiense Superior. Est...

Orbital pacing and secular evolution of the Early Jurassic carbon cycle

“…Several medium-amplitude CIEs observed in Mochras have previously also been identified in other δ 13 C records covering shorter stratigraphic intervals from geographically widespread sections, and obtained from different carbon substrates, demonstrating that these shifts do not represent a local phenomenon restricted to the Cardigan Bay Basin. In the Sinemurian, for example, a positive CIE in the turneri zone is also recorded in δ 13 C TOC on the Dorset coast of the United Kingdom (20) and in western North America (23). A negative CIE in the obtusum-oxynotum zones is also present in previously published δ 13 C records of bulk organic matter, belemnites, and terrestrially derived palynomorphs from Lincolnshire, United Kingdom, and the δ 13 C TOC records from Robin Hood's Bay, United Kingdom and Sancerre-Couy, France (15,21,24,42).…”

“…Stratigraphically expanded shifts were recorded at the Sinemurian-Pliensbachian boundary (8)(9)(10)(11)(12)(13)(14) and the upper Pliensbachian margaritatus and spinatum zones (10,15,16). Furthermore, multiple stratigraphically less extended short-term δ 13 C shifts of ∼0.5 to 2‰ magnitude have been recognized throughout the Hettangian (17)(18)(19), in the Sinemurian (17,(20)(21)(22)(23)(24), and Pliensbachian (10,11,16,22,(25)(26)(27)(28), where they are recorded as individual shifts or series of shifts within stratigraphically limited sections. Some of these short-term δ 13 C excursions have been shown to represent changes in the supraregional to global carbon cycle, marked by synchronous changes in δ 13 C in marine and terrestrial organic and inorganic substrates and recorded on a wide geographic extent (e.g., refs.…”

Sinemurian–Pliensbachian calcareous nannofossil biostratigraphy and organic carbon isotope stratigraphy in the Paris Basin: Calibration to the ammonite biozonation of NW Europe

“…1 ‰) as compared to other, more prominent features recorded higher up in the core. Therefore, we do not define here any significant event in most of the Sinemurian although some trends may potentially correlate to recent studies on organic matter in UK and North America (Jenkyns and Weedon, 2013;Porter et al, 2014). Significant organic carbon isotope events can be defined with more confidence in our record from the upper Sinemurian to the top of the Pliensbachian.…”

Palaeoecological insights on Toarcian and lower Aalenian calcareous nannofossils from the Lusitanian Basin (Portugal)

“…to pre-tempestite conditions, hence enabling the rebuilding of the photic zone ecosystem, as positive δ 13 C excursions recorded across the Early Toarcian are interpreted as reflecting oceanic primary productivity and preferential light carbon uptake in photosynthesis (e.g., Jenkyns and Clayton, 1986;Kump and Arthur, 1999;Jenkyns et al, 2002;Porter et al, 2014).…”